The construction of large, impermeable, thermoplastic membranes is typically accomplished by transporting rolls of bulk thermoplastic materials to a work site. Individual sheets are unrolled, cut to length, and then positioned on a supporting structure so that the edges of the sheets may be welded or fused together in situ to form a singular unitary membrane. Such membranes are used as roof coverings, lagoon and reservoir liners, and also as large tarpaulins, among others.
The welding or fusing of the thermoplastic sheets, in the field, by means of portable welding machines is conventionally achieved by abutting or overlapping the edges of the individual sheets. Heat is applied to the edges until they have changed from a solid to a tacky viscous fluid state. The heated portions are then compressed together while they are hot so that the sheets, upon cooling, become a single unitary membrane consisting of a multiple of fused sheets.
The welding together of individual sheets in situ is accomplished in the prior art in one of two ways. The first is where the sheets are initially laid out with their edges overlapping. In that case an upper and a lower sheet, referred to as opposing sheets, are separated and a heat source is interposed between them. The heat source heats a bottom portion of the upper sheet along its edge, and a top portion of the lower sheet along its edge. The two heated portions are then compressed together while they are hot to form a welded membrane. For the other way, the individual sheets, referred to as primary sheets, are laid out with either abutting or overlapping edges and the sheets are fused by welding a strip or tape of thermoplastic material along the butt or overlap, forming a seam. In that case, three individual sheets are involved, the two primary sheets and a strip or tape. The strip or tape is fused equally to both primary sheets.
Conventionally, the tape or strip is applied from the top and placed onto the two primary sheets below which are lying on the supporting structure. In that case, the tape or strip is the upper sheet. The heat source is interposed between the tape or strip and the primary sheets below. The bottom of the tape or strip is heated along with the top of the edges of the primary sheets. The strip is the opposing sheet relative to the primary sheets. The weld is completed following the compression of the heated portions into a unitary membrane. In either case, a heat source is interposed between an upper and a lower sheet or sheets and the primary or opposing sheets are heated in preparation for the compression. The upper and lower sheet or sheets are hereinafter referred to as opposing sheets for all cases. In the latter type of cases, tape or strip is hereinafter referred to as strip.
Machines of the referenced type are generally motor driven, self-propelled machines in which a drive motor is connected through a drive train to one or more drive wheels. A heating means, commonly a hot-air blower, and a sheet handling means are provided on the machine for guiding at least one of the opposing sheets through the machine and past the heating means as the machine travels along. The edges of the opposing sheets are heated and then laid or placed together while hot. The laminate of hot sheets then passes through a compression means and out of the machine as the weld is completed automatically.
In conventional portable welding machines, a single heat or temperature source heating means heats the opposing sheets until a fluid or semi-solid state has been achieved to allow for the compression and fusion of the sheets. When using a single heat means which operates at a constant temperature a problem is presented when different thermoplastic sheet materials are to be fused. A similar problem is presented when the same or different thermoplastic sheet materials are to be fused but the sheets are of different thicknesses.
Thermoplastic sheet materials have a range of thermal properties. For example, a typical molding temperature for plasticized vinyl begins at 285.degree. F. and that for polypropylene typically begins at 350.degree. F. It is often desirable to fuse two such different materials, particularly when called upon to repair a tear or rupture in an existing membrane.
The quantity of heat required to fuse thermoplastic materials varies with the thicknesses of the sheets to be fused. It is often desirable to fuse sheets of different thicknesses, particularly when using a strip to fuse two primary sheets. When fusing sheets of different thicknesses, the different thermal requirements of each of the opposing sheets lead to different temperature or heat means differential output requirements for each of opposing sheets. The differential requirements are compounded when the sheets are of both different thermoplastic materials and different thicknesses.
A similar heat differential problem exists when similar or different individual sheets have been exposed to different ambient environments prior to the time when welding begins. This situation typically occurs when the individual sheets, which are to become a roofing membrane, are laid out in the sunlight and reach high temperatures due to natural solar radiation and the welding is started by using a strip of material previously stored in a sheltered location and thus is at ambient air temperatures.
Conventionally, an operator uses a single heat source operating at a single temperature until the opposing sheets are somewhat fluidized. In other words the different thermal requirements of the opposing sheets are generally disregarded in conventional systems. The result of this technique, typically achieved by single temperature hot-wedge or hot-plate machines, is often a faulty weld caused by the over curing or hardening of the sheet with the lower thermal requirements. Since a hardened membrane cracks and fails, this result is undesirable. A similar result is achieved by single heat source hot air machines where the operator either manually distributes the heat between the opposing sheets using intuition, experience, or skill, or applies heat generally as is done with the hot-wedge machines.
The aforementioned temperature and heat differential problems are solved by the present invention. The present invention is limited to electric radiant heat sources in order to overcome other environmental problems typically encountered in the field when using portable hot air machines. The hot air machine, while amendable to a dual heat source welder, is too sensitive to outdoor wind conditions to be a practical solution to the aforementioned problems. The natural wind often blows away the hot air leading to ineffective or spotty heating and welds.
A second problem is presented when the opposing heated sheets are compressed together with conventional portable welding machines. Following the heating of the sheets with a conventional machine, the sheets are fused, by a compression means, into a unitary membrane. The conventional compression means consists of a single roller or wheel and thus the sheets are compressed once and essentially instantaneously as the machine travels on. The region of compression action is in the shape of a line transverse to the major axis of the weld and beneath the roller. That line region then travels along the seam automatically at the linear velocity of the machine.
The conventional compressing technique creates problems. Impurities, particularly bubbles, which are trapped between the sheets prior to the compression, have a tendency to be distributed along the seam or are trapped within the weld without lateral expulsion or dissolution. This creates undesirable welds containing impurities. The problem is compounded when combined with the aforementioned thermal differential requirements. In cases where a bubble is trapped in the weld and the upper opposing sheet has been overheated, the bubble forms a hardened dome, a burned-bubble, which is easily fractured.
There is therefore a need for a portable machine that provides compression of the hot sheets for an extended period of time to decrease the incidence of impurity entrapment in the weld, particularly with respect to bubbles. This problem is solved by the present invention.